random: Add a note to avoid misconception about virtio_rng_read

Signed-off-by: Raphael S. Carvalho <raphaelsc@cloudius-systems.com>
bsd/random: Fix entropy estimation on data from RANDOM_PURE_VIRTIO sources

Currently, the code that harvests entropy from hardware sources of entropy
unconditionally consider only half the bytes as entropy data.

For example, when reading N bytes from virtio-rng only N/2 bytes would be
taken into account by Yarrow when estimating entropy, which isn't true.
As a result of that, reseeding the random devices requires two times the
amount of data than actually needed. Then meaning that entropy data from
virtio-rng is being unnecessarily wasted.
Applying this patch would make reseeds happen with more frequency, about
two times faster, so I'm dividing by two the frequency in which the random
thread wakes up, thus balancing things.

Reseeds don't happen on every time the thread feeds entropy to Yarrow as it
was designed to accumulate entropy, and perform reseed after a determined
threshold is reached.

Let's fix the former problem by making the division when estimating entropy
a conditional decision.